Which of the following is NOT a function of a bicycle helmet?

A bicycle helmet serves several key functions designed primarily for safety and performance. The primary role of a helmet is to reduce impact force on the head during a fall or collision. This is achieved through the helmet's construction, which includes a hard outer shell and an inner foam layer that absorbs and dissipates energy from impacts.

Another important function of a helmet is to protect against flying debris, such as rocks or branches, that could cause injury while riding, especially in areas where trails or roads may be unpaved or surrounded by nature. The design often incorporates a visor or extended coverage to help shield the rider's face and eyes.

Improving aerodynamics while riding is also an essential function, particularly for competitive cyclists. Helmets are designed to streamline airflow around the rider's head, reducing drag and enhancing speed.

Providing warmth in cold weather is not a primary function of a bicycle helmet. While some helmets may offer thermal insulation, it is not a safety feature nor a fundamental purpose of the helmet’s design. Helmets are primarily engineered for protection against impacts and hazards encountered while riding, rather than for temperature regulation.